Currently, a form of using computer resources (for example, the server, storage, application, and so on) via a network, so called cloud computing is practiced.
In cloud computing, the user is only required to prepare a minimum connection environment including a terminal such as a personal computer and the like and a browser that operates on the terminal and the like, alleviating troubles such as maintenance of the apparatus and data management.
The service provided in the form of cloud computing is called cloud service.
A provider of the cloud service presents the target value (Service Level Objective: SLO) of the availability of the system, and provides service in accordance with it.
The operation of the system to provide the cloud service involves updating works such as application of a security patch, upgrading and the like. Such updating works are carried out by the cloud service provider in cloud computing.
A system that provides the cloud service has components consisting of hardware components and software components. Hardware components are hardware such as the server and network device for example. Meanwhile, software components are software such as the Virtual Machine Monitor (VMM), Virtual Machine (VM), Operating System (OS), application and the like.
Some combinations of these components cause troubles, but information on combinations with which troubles occur are not readily provided by the vendor of each component.
Therefore, it is difficult to avoid combination of components with which troubles occur, and a trouble occurs sometimes when updating a component, causing failure to attain the SLO.
Meanwhile, as a method to avoid deterioration of availability, that is, the operation performance of a service, there is a method to prepare a testing environment, check presence/absence of a trouble by performing the updating in the testing environment, then to perform the updating in the actual environment. However, such a method has a problem that the cost is very high, since a complicated configuration that is similar to the actual environment is prepared to check the trouble.
When failure to attain the SLO occurs due to a trouble caused in updating a component, the provider of the cloud service has to pay a penalty, reducing its income. In addition, a cloud service that can only offer low availability causes user withdrawal.
Therefore, it is preferable that the availability does not fall below the target value.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2004-259110    [Patent Document 2] Japanese Laid-open Patent Publication No. 2005-107803    [Patent Document 3] Japanese Laid-open Patent Publication No. 2010-128581